The present invention is directed to systems, methods and apparatus for imaging and ablating surface contours. In particular, the invention relates to methods and apparatus for generating direct images of the anterior region of the eye. The present invention is particularly useful for generating silhouette images of the ablated region of the cornea during or immediately following a laser ablation procedure, such as photorefractive keratectomy (PRK), phototherapeutic keratectomy (PTK), laser in-situ keratomileusis (LASIK) or the like. The silhouette images include ablation profiles of the cornea that can be used to refine laser ablation procedures by tailoring the ablation process to match the actual ablation properties of the human eye.
Ultraviolet and infrared laser based systems and methods are known for enabling ophthalmological surgery on the external surface of the cornea in order to correct vision defects. These procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of an anterior stromal tissue from the cornea to alter its refractive power. In ultraviolet laser ablation procedures, the radiation ablates corneal tissue in a photodecomposition that does not cause thermal damage to adjacent and underlying tissue. Molecules at the irradiated surface are broken into smaller volatile fragments without heating the remaining substrate; the mechanism of the ablation is photochemical, i.e. the direct breaking of intermolecular bonds. The ablation penetrates into the stroma of the cornea to change its contour for various purposes, such as correcting myopia, hyperopia, and astigmatism.
In such laser based systems and methods, the irradiated flux density and exposure time of the cornea to the laser radiation are controlled so as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea. To that end, ablation algorithms have been developed that determine the approximate energy density that must be applied to remove a certain depth of tissue from the cornea. At ultraviolet wavelengths, for example, an energy density of 1 joule/cm.sup.2 will typically ablate corneal tissue to a depth of about one micron when applied in a series of pulses of about 100 to 400 millijoules/cm2. Accordingly, the ablation algorithms are tailored for each procedure depending on the amount and the shape of corneal tissue removal required to correct the individual refractive error.
Although present laser ablation algorithms are relatively accurate, many of these algorithms assume that tissue ablation is uniformly related to irradiance within the treated zone. Recent studies of corneal topography following PRK, however, have determined that, in some instances, there is a central area of undercorrection called a "central island" for large diameter areas of cornea exposed to a uniform laser irradiance. The term "central island" is generally defined as a central area of corneal ablation that appears to be flattened less than the surrounding ablated area. In contrast to central islands, central overcorrection and peripheral undercorrection within the ablation zone have also been reported following PRK procedures. See e.g., Lin DDTC, et al. Corneal Topography Following Excimer Phototrefractive Keratectomy For Myopia. J. Cataract Refract. Surg. (1993) 19:149-154. Although mild central topographic changes may not have a large bearing on visual function, some patients with clinically significant central islands can experience visual abnormalities including reduced best corrected visual acuity, monocular diplopia and image ghosting.
Investigators have studied modified ablation algorithms as methods to improve the quality of patient vision and post operative corneal surface. See e.g., Seiler T., et al. Complications of myopic photorefractive keratectomy with the excimer laser. Ophthalmol. (1994) 101:153-160. It is believed that some common surface abnormalities (such as central islands) may be at least partially corrected by changing the ablation algorithm. For example, the ablation algorithm may be altered to provide additional laser pulses centrally to offset the effects of central islands.
Currently, the accuracy of modified ablation algorithms is determined from experimental ablation data and post-operative clinical data based on topographic measurement of the healed corneal surface. The healed cornea, however, has been covered by tear films and an epithelium layer with an average thickness of 50 .mu.m. Consequently, topographic measurement of the healed anterior corneal surface after the laser ablation procedure may not accurately portray the true shape of corneal ablations. Accordingly, direct measurement of the ablated corneal surface would be extremely desirable (if not necessary) to understand the effects of changes to laser ablation algorithms on the ablated corneal shape.
Corneal topographic analysis using photokeratoscopic or videokeratographic methods provides objective measures of the quality of the healed anterior corneal surface following ablation procedures, such as PRK, PTK and the like. Current measurement devices, termed videokeratoscopes or corneascopes, typically employ several concentric rings or multiple discrete light sources to reflect a luminous object of known dimension from the cornea. The size of the cornea-reflected images of this object are then measured with photographic or electrooptical recording methods to compare the shape of the cornea with a theoretical spherical shape. If the cornea is spherical, for example, the reflected images of these ring-shaped objects are equally spaced, continuous, concentric ring-shaped patterns. If the cornea has surface defects, or is not precisely spherical, the resultant ring images will be less equally spaced, or they will have a different shape, e.g., elliptical.
One of the drawbacks with many current methods of topographic analysis is that these methods typically treat the cornea as a close approximation to a convex sphere, and they require a specularly reflecting surface. Since the cornea is not precisely spherical, the results of the measurement generally depend on where the non-spherical cornea measurement is taken. Moreover, the corneal surface is not specularly reflective immediately following a laser ablation procedure, such as PRK. Another drawback is that videokeratoscopes do not measure the actual cornea topography, but merely measure the "average" radius. The central few millimeters of the cornea, which is very important optically, cannot be directly evaluated with these devices. Another drawback with these methods is that they are not capable of precisely measuring extremely small surface changes on the anterior corneal surface, e.g., on the order of 20 microns or less. The non-uniform spatial distribution of tissue ablation in laser procedures, however, is generally on the order of about 5-10 microns or less (i.e., about 10-20 percent of the intended ablation depth). Therefore, current methods of topographic analysis are not as accurate for measurements of the spatial variation of tissue removal as one would desire.
It would also be desirable to accurately measure the shape of the ablated region of the cornea during or immediately following the ablation procedure (i.e., prior to healing of the corneal tissue and regrowth of the epithelium layer). This would allow a direct measurement of the ablated surface without the tear films or the epithelium layer interfering with the accuracy of the measurement. In addition, this would allow the surgical team to characterize the profile of ablated corneas and to determine the spatial variance of tissue ablation rates during the surgical procedure, which may afford the opportunity to provide in situ feedback to the surgeon.
Unfortunately, it is extremely difficult to accurately measure the ablated surface of the cornea during or immediately following a surgical procedure with current techniques. One reason for this difficulty is that the epithelium is removed from the cornea prior to PRK to expose the diffusely reflective stromal corneal surface. Thus, keratometry techniques based on reflection are unsuitable. Moreover, although corneal topography utilizing videokeratography can be performed immediately after PRK when artificial tears have been instilled, it is the shape of the tear film and not the ablated cornea which is measured. In addition, the use of corneal topography techniques to assess the acute shape of PRK ablations also suffers from the inability of these instruments to accurately measure surfaces with an abrupt change in curvature.
Projective techniques utilizing rasterstereographic imaging such as the PAR Technologies Corneal Topography System (U.S. Pat. No. 4,995,716) have been used on deepithelialized and freshly keratectomized corneas. These rasterstereographic systems project a grid on the cornea which is then imaged by a video camera, digitized and analyzed to produce a tabulation of corneal elevations versus corneal diameters. Since the cornea is a transparent member which is non-diffusing to light, however, a grid projected onto the cornea is not sufficiently visible. Thus, a diffusing material, such as talcum powder or a liquid film of fluorescent dye, is typically applied to the cornea to provide a surface on which By an image can be visualized. These powders or films degrade the accuracy of the measurement because it is the shape of the powder or film (and not the cornea) that is measured.
What is needed, therefore, are improved methods and apparatus for measuring the outer profile of the eye. In particular, these methods and apparatus should be capable of directly measuring the depth of corneal ablation to the order of about 5.0 to 10.0 .mu.m or less in order to evaluate the spatial variance of tissue ablation rates. Further, it would be particularly desirable if these methods and apparatus were capable of directly measuring the ablated region of the cornea during or immediately following the laser ablation procedure to provide an accurate profile of the ablated corneal surface (without adding a liquid film or powder to this surface). Such information may be used to modify the ablation algorithm based on differences between the theoretical and directly measured ablation profiles.